the mechanism to level a sight could be a simple as barrel mounted bearings with enough counter weight to keep the scope vertical and enough friction to resist swinging. A LER scope would be needed so the scope could rotate around the bore's axis.

I wonder if I could stabilize a scope with a gyroscope?

__________________Hundreds of years from now, it will not matter what my bank account was, the sort of house I lived in, or the kind of car I drove... But the world may be different because I did something so bafflingly crazy that my ruins become a tourist attraction.